













r 







•i", -€ 
^^'O* 

















.'^^'^ 




^oV^ 






•^-^ • 




,^o^ 






















5 









.^''^ 














.^'^^ 







^^-n^. ^-^ 



• ■«' 



.^ .,. 





















• a 



^Cr^9^ 




0*^ r'^.^' 



^""^ 











'^ A*' 






♦ aV "^^ 



1^ 



V-S' 






0' 



'bV- 



.^S'' 



"^ 'WI^W» A^-^ ^^6^= c'^^rv o^^^^» aV«^. - 



^^-n^^ 



r^ 



^ 











«^ xV ^^ - * * • 4 ' 




















IIIIIIMMIIMMMIMnilllMMIMIIIIIIMMIMIIIII 



IC 9248 



BUREAU OF MINES 
INFORMATION CIRCULAR/1990 

y93 



Dust Control In Coal Preparation and 
Mineral Processing Plants 

By Edward F. Divers and Andrew B. Cecala 




U.S. BUREAU OF MINES 
1910-1990 

THE MINERALS SOURCE 



^AU OF >^' 



Mission: Asthe Nation's principal conservation 
agency, the Department of the Interior has respon- 
sibility for most of our nationally-owned public 
lands and natural and cultural resources. This 
includes fostering wise use of our land and water 
resources, protecting our fish and wildlife, pre- 
serving the environmental and cultural values of 
our national parks and historical places, and pro- 
viding for the enjoyment of life through outdoor 
recreation. The Department assesses our energy 
and mineral resources and works to assure that 
their development is in the best interests of all 
our people. The Department also promotes the 
goals of the Take Pride in America campaign by 
encouraging stewardship and citizen responsibil- 
ity for the public lands and promoting citizen par- 
ticipation in their care. The Department also has 
a major responsibility for American Indian reser- 
vation communities and for people who live in 
Island Territories under U.S. Administration. 



Information Circular 9248 



Dust Control in Coal Preparation and 
IVIineral Processing Plants 

By Edward F. Divers and Andrew B. Cecala 



UNITED STATES DEPARTMENT OF THE INTERIOR 
Manuel Lujan, Jr., Secretary 

BUREAU OF MINES 
T S Ary, Director 



^^ 



\ 



^ 







<o 



Library of Congress Cataloging in Publication Data: 



Divers, Edward F. 

Dust control in coal preparation and mineral processing plants / by Edward F. 
Divers and Andrew B. Cecala. 

p. cm. — (Information circular / Bureau of Mines; 9248) 

Includes bibliographical references. 

1. Coal preparation plants — Dust control. 2. Ore-dressing plants — Dust control. 
I. Cecala, Andrew B. II. Title. III. Series: Information circular (United States. 
Bureau of Mines); 9248 



TN295.U4 



[TN816] 622 s--dc20 [622'.83] 



90-1528 
CIP 



CONTENTS 

Page 

Abstract 1 

Introduction 2 

Background 2 

Coal preparation 2 

Mineral processing 2 

Dust sampling instruments 2 

Ventilation 3 

Whole plant 3 

Local exhaust 3 

Air cleaners 4 

Baghouse-type dust collectors 4 

Scrubbers 5 

Electrostatic precipitators 6 

Control at source 7 

Water sprays 7 

Good housekeeping 7 

Personal protection devices 8 

Dust helmets 8 

Respirators 9 

Special problems 9 

Pipe and duct clogging 9 

Control room dust 10 

Conclusions 11 

References 11 

ILLUSTRATIONS 

1. Effects of bulk-loading outside on worker's exposure inside mill 3 

2. Wall exhaustor — typical layout 4 

3. Baghouse-type dust collector 4 

4. Small-diameter cyclone type scrubber 5 

5. Flooded fibrous bed-type scrubber 5 

6. Venturi-type scrubber — horizontal arrangement 5 

7. Venturi-type scrubber — vertical arrangement 6 

8. Electrostatic precipitator 7 

9. Increase in worker's dust exposure from broom sweeping 8 

10. High capacity remote vacuum system 8 

11. Dust helmet 8 

12. Disposable-type respirator 9 

13. Positive pressure-type respirator 9 

14. Settling type drop-out chamber 10 

15. Baffle type drop-out chamber 10 

16. Centrifugal type drop-out chamber 10 

TABLE 

1. Respirable dust test results 6 





UNIT OF MEASURE ABBREVIATIONS USED IN THIS REPORT 


Btu/h 


British thermal unit per 


hour 


hp 


horsepower 


cfm 


cubic foot per minute 




in 


inch 


ft 


foot 




inHjO 


in of water (pressure) 


ft' 


cubic foot 




mg/m' 


milligram per cubic meter 


ft/min 


foot per minute 




min 


minute 


gpm 


gcdlon per minute 




pet 


percent 


h 


hour 




st/h 


short ton per hour 



DUST CONTROL IN COAL PREPARATION 
AND MINERAL PROCESSING PLANTS 



By Edward F. Divers^ and Andrew B. Cecala^ 



ABSTRACT 

This U.S. Bureau of Mines report briefly evciluates the advjmtages and disadvantages of basic dust 
control techniques presently used by U.S. coal preparation and mineral processing plants. These include 
ventilation, baghouse-type collectors, wet scrubbers, electrostatic precipitators, source control, sprays, 
good housekeeping, and personal protection devices. Two specific problems in these types of operations 
are also considered: dust collector system duct clogging, and control room dust control. 

Information provided in this report results from dust control research projects conducted by the 
Bureau at various coal preparation and mineral processing plants over the past decade to reduce 
workers' dust exposure. These studies indicate that plant ventilation systems normally provide the most 
cost effective method for dust control. Baghouses and scrubbers were also effective in specific 
appUcations, and examples of each are given. In extreme dust conditions, personal protection devices, 
such as respirators or the dust helmet, can also be highly cost effective. 

Mining engineers, Pittsburgh Research Center, U.S. Bureau of Mines, Pittsburgh, PA. 



INTRODUCTION 



Coal preparation and mineral processing plants can 
present a wide vciriety of dust problems. Fortunately, 
the problems in both types of plants can be reduced or 
resolved by the same basic techniques previously 
mentioned. This U.S. Bureau of Mines report provides 



information to operators considering various options for 
dust control. Its conclusions are primarily based on the 
field experience of Bureau personnel and plant operators 
and can be applied to most, if not all, plant dust problems. 



BACKGROUND 



COAL PREPARATION 

There are approximately 500 coal preparation plants in 
the United States. The number of operators in each plant 
ranges from about 3 to 10 per shift; raw capacity ranges 
from roughly 250 to 1,250 st/h. 

A recent Bureau survey of 21 coal preparation plants 
showed that one-third had highly localized respirable dust 
levels. These were dust levels in given specific areas of 
the plant and not exposure levels of plant personnel. No 
relationship was found between dust concentration and age 
or capacity of the plant. Some old plants show 1 mg/m^ 
or less in all areas. Some new plants had respirable dust 
concentrations up to 11 mg/m^ in specific areas. In 
general, these areas only had temporary occupancy, seldom 
exceeding several hours. High dust concentrations were 
closely associated with dry coal. With a few exceptions, 
these high dust concentrations were found in poorly 
ventilated areas on the ground floor. Excluding venti- 
lation, few plants use operating dust control devices. Silica 
(quartz) analysis made on gravimetric samples from coal 
preparation plants show that the percent siHca was usually 
below Federal limits. 

MINERAL PROCESSING 

There are well over 1,000 mineral processing plants in 
the United States. While dust concentrations in these 
plants are generally much lower than in coal preparation 
plants, the associated respiratory problem can be more 
severe due to siUca. In recent years. Bureau studies have 
concentrated on those mineral processing plants having 
silica problems; the percent silica on gravimetric samples 
varies from to over 90 pet. The problem can be espe- 
cially severe in the silica sand industry where respirable 
silica usually is in the 80 to 90 pet range. When silica is 
present, the FederaJ dust standcu^d is tightened below the 
usual 2 mg/m^. 

Unlike coal preparation, mineral processing plants 
usually process the product completely dry because of 
screen and duct clogging problems associated with wet 
products. When products JU"e hygroscopic, the possibility 



for any type of wet processing is excluded. Another major 
difference is that many workers at mineral processing 
plants spend the majority of the workday at a specific 
location, which is not common in coal preparation plants. 
This can extend ventilation to year-round use, and fixed 
worker locations can be economically heated by infrared 
devices. Although they are not generally recommended 
due to costs, the use of steam, foams, and water additives 
(surfactants) may be appropriate for some mineral 
processing plants. Water sprays can also be an effective 
seasonal or occasional dry product (such as strip coal) 
control technique for both types of plcmts. 

DUST SAMPLING INSTRUMENTS 

Two types of dust sampling equipment were used 
during these evaluations: gravimetric dust samplers and/or 
instantaneous dust monitors called real-time aerosol 
monitors (RAM) (1)} 

Gravimetric dust samples have been the primary means 
of determining respirable dust concentrations used by the 
Mine Safety and Health Administration (MSHA) since 
dust monitoring became a mandated practice as estab- 
Ushed by the 1969 Federeil Mine Health and Safety Act. 
Although gravimetric sampling is performed for all com- 
pUance testing, it is not the most advantageous technique 
for many studies because of the time required to obtain 
dust data. 

The instantaneous RAM-1 dust monitor built by 
Monitoring Instruments for the Environment (MIE), Inc., 
Bedford, MA overcomes the drawbacks of gravimetric 
sampling. The RAM-1 unit instantaneously calculates 
respirable dust concentrations. However, the RAM-1 
cannot distinguish the quartz concentration of the dust. 

Visucd observation can be an effective technique for 
estimating dust concentrations, even though water mist can 
cause problems. As a general guideUne, respirable dust 
concentrations below 2 mg/m' will not create visibiUty 
problems or hcdoing around lights; these are especially 

ntalic numbers in parentheses refer to items in the list of references 
at the end of this report. 



evident above 5 or 6 mg/m'. Visibility problems beyond 
50 or 60 ft typically start at 8 to 10 mg/m^, and distinct 
visibility problems beyond 20 to 30 ft occur above 15 or 20 



mg/m^. At these high respirable concentrations, coal dust 
odor can be detected and rapid accumulation of all types 
of larger dust p2u-ticles on most surfaces is evident. 



VENTILATION 



WHOLE PLANT 

Ventilation can be an excellent low-cost dust control 
technique for all plants during mUd weather. Plants using 
effective ventilation systems have low dust concentrations, 
with peaks generally below 2 mg/m^ respirable. These 
systems consists of powered exhaustors (fans) or gravity 
ventilators in the roof, open access doors, and open-close- 
type wall louvers at grade. Due to a sUght increase in 
initial plant costs, ventilation systems are not built into 
most plants. However, since ductwork is not required, 
subsequent addition of a ventilation system can be 
relatively inexpensive, about $1 per cubic foot per minute 
for a powered roof exhaustor system with adjustable 
louvers at grade. Powered roof exhaustors should be sized 
to provide between 6 to 12 air changes per hoiu". Where 
dust concentrations are known or safely assumed to be 
low, 6 to 8 air changes per hour can be used. 



Air changes per hour 



Total volume exhausted per hour 
Total volume of ventilated building 



However, long-term, high dust concentrations, above 
6 mg/m^, may not be adequately controlled by ventilation 
alone. In these cases, primary efforts should be directed 
to control at the dust source, or using dust collectors such 
as baghouses or scrubbers. 

Design guidelines for the installation of plant 
ventilation systems am be found in the "ASHRAE 
Handbook of Fimdamentals" or their systems handbook 
published by the American Society of Heating, 
Refrigeration, and Air-Conditioning Engineers, Inc., 
Atlanta, GA, or "Industrial Ventilation" published by the 
Committee on Industrial Ventilation, Cincinnati, OH. 

For ventilation to be effective as a dust control 
technique, it is critical that the design be capable of 
supplying relatively clean makeup air to the system. If air 
supplied to the ventilation system is contaminated, the 
ventilation system can be ineffective and may increase a 



worker's dust exposure. This occiu-red at a mineral 
processing plant where trailer trucks were being bulk- 
loaded outside a mill building (fig. 1). The dusty air 
generated from the bulk-loading process was drawn into 
the mill as makeup jiir for a ventilation system; worker's 
dust exposure was increased 147 pet over previous 
(normal) levels diuing the buUc-loading. Inlet louvers 
should not be placed neai outside dust or other 
contaminant sources. 

LOCAL EXHAUST 

Local exhaust through wzdl exhaustors, fans, and other 
devices can be a highly cost-effective technique for 
removing dust from specific, generally small, cueas. This 
is frequently done with wall-mounted propeller fans 
properly protected cmd mounted close to the dust source , 
usually without ductwork (fig. 2). Fan volumetric capac- 
ities generally range from 10,000 to 25,000 cfm or more, 
with motors from 1 to 5 hp. The exhaustors aie controlled 
by operation of the dust-generating soiu-ce, such as a 
crusher, emd/or mjmual control. Because strong outside 
winds can negatively affect the performance of exhaustors, 
their placement should normally be on the nonwindward 
(lee) side of buildings. 



Bulk loading outside 




8 12 

TIME, min 

Figure 1. -Effects of bulk-loading outside on worker's 
exposure inside mill. 



Airflow 




Guard 
(outside) 



and boot 



Manual shutter 
(aluminunn) 



Figure 2.-Wall exhauster — typical layout 



AIR CLEANERS 



BAGHOUSE-TYPE DUST COLLECTORS 

Baghouses collect airborne dust by filtration through 
special cloths, somewhat similar to a home vacuum 
sweeper. The collected dust is normally shaken auto- 
matically from the filters into a hopper (fig. 3). Baghouses 
have several advantages, especially for mineral processing 
plants: they offer high dust collection efficiency that is 
essentially independent of mineral type, cmd they do not 
use water. When in-plant space is not available, outside 
installations are generally successful. Disadvantages are 
size (especially height), duct routing and cost, handling of 
the collected dust, and frequent problems when excessive 
moisture or water is in the air. Initial cost generailly 
exceeds $4 per cubic foot per minute. Nevertheless, 
baghouses are especially recommended for those plamts on 
a reduced silica standard. Where very high dust collection 
efficiency is required, use a baghouse. 

Visible dust in the exhaust plume always indicates 
problems in baghouse systems. For example, at one coal 
preparation plant a 38,000 cfm outside unit discharged to 
the atmosphere. A visible plume was emitted from its 
exhaust and respirable dust sampling showed a concen- 
tration of 5.0 mg/m^. This was caused by torn bags. 
Properly designed and maintained systems will show less 
than 1 mg/m in the exhaust plume, and no visible dust. 

Another baghouse system was located on the second 
floor of a 5-year old plant. Its capacity was approximately 
15,000 cfm, and it eilso exhausted to the outside. A simple 



manually operated spUtter was installed in the fan 
discharge duct that also allowed inside discharge during 
cold weather. Since the system was adequately main- 
tained, exhaust concentrations averaged 0.70 mg/m^. 
Inside discharge during severe cold weather saved over 1 
milhon Btu/h by reducing makeup air heating cost. Inside 
discharge should not be used when sihca dust is present. 



Collector-fan 



Clean air 
out 




Solid floor 
perforated 
at bag inlet 



Figure 3.-Baghouse-type dust collector. 



Silica dust presents a serious dust hazard and requires the 
plant to meet a lower Federal dust standard. 

Baghouses can achieve very high respirable dust 
collection efficiency, well above 99 pet with special bag 
filter material. This high efficiency cannot be readily 
achieved by scrubbers. However, scrubber efficiency can 
be adequate for most coal preparation appUcations. 

SCRUBBERS 

Scrubbers frequently offer another practical control 
method, especially for coal preparation plant dust control. 
They collect airborne dust by highly turbulent collision 
with water droplets or a wetted surface, the water then 
being separated from the airstreemi. The water consumed 
does not present problems to most coal preparation plant 
operators, but can make them imsuitable for minereds 



processing. Dust collector efficiency can be sufficient to 
allow the scrubbed air to be discharged back into the 
plant, and they £ire small enough to allow them to be 
retrofitted into operating plants. Typical components 
consist of a 25,000 to 50,000 cfm fan generally operating 
between 6 to 10 in HjO, the scrubbing device, water sprays 
at a flow rate between 0.5 and 3 gpm per 1,000 cfm of 
airflow, a water droplet eliminator and duct work to 
various dust sources. Initial costs are $2 to $3 per cubic 
foot per minute, and mziintenance is uncomplicated. 

Disadvantages are fan power cost and fan noise, 
potential clogging, and cold weather freezing problems. 

Three commonly used types of scrubbers were eval- 
uated for this work: the small-diameter cyclone (fig. 4), 
the flooded bed (fig. 5), and the venturi (fig. 6) (2). The 
primary purpose was to determine if the cleaned air could 
be safely dischairged back into the plant. 



-Turning vanes 




Airflow 
U^ to fan 



Figure 4.-Small-diameter cyclone type scrubber. 



-Spray Blade type 

nozzles rnist eliminator-, 

''^ [ pLj^ Airflow 
_T^ to fan 




/ 



' — ^ Drain 
Figure 5.-Fiooded fibrous bed-type scrubber. 



Long 
transition 



Blade type mist 



eliminator-i transition- 



Centrifugal 
fan 



Dusty air 
inlet duct 




Figure 6.-Venturi-type scrubber — horizontal arrangement 



Dust sampling results showed that all three can do jui 
adequate job of dust control at the pickup locations. 
Results at the vicinity of the scrubber fiui discharge were 
determined (table 1). 

Table 1 .-Respirable dust test results' 
(8-h gravimetric averages at scrubber) 



Scrubber type 


Discharge, 
mg/m^ 


Efficiency, 
pet 


Small-diameter cyclone . . . 

Flooded fibrous bed 

Venturl 


2.47 
1.12 
3.09 


88 
95 
85 



Mnlet was 21 for all three types of scrubbers. 

The flooded bed scrubber was more than twice as 
efficient as the small-diameter cyclone, allowing unlimited 
exposiu-e time. The results are consistent with previous 
Biu-eau studies comparing both scrubber types (3-4). 
However, both scrubbers clogged with coal particles, 
creating imacceptable maintenance problems. Air pressure 
drop across the flooded bed panel increased from 3.8 to 
6.5 in of water during 9 h of typical operation due to 
clogging. The cyclone panel increased about hadf of this 
2.7 in rise, i.e., 1.37 in, under identical conditions. Proper 
operation of these scrubbers is highly dependent upon a 
steady and uniform application of spray water. Clogged 
and partly clogged sprays decrease the dust collection 
efficiency, and increase the pressure buildup across both 
cyclone and flooded bed types. 

Aucdysis of the used flooded bed panels showed that 
clogging was primarily caused by large particulate (up to 
1/16-in) embedded in the mesh. This indicates that the 
dust inlet duct was located too close to the dust source. In 
most underground applications, the flooded bed panels are 
cleaned once per shift by drying and shaking, or by direct 
flushing with a hose. 

The venturi showed no indication of clogging. Its 
efficiency was limited by the fan pressure capabihty in this 
installation that only allowed a maximimi of 6 in HjO 
gauge differential pressure across the ventiu-i throat. The 
dust collection efficiency of a venturi scrubber is essen- 
tially unlimited, and primarily dependent on fan pressme 
capability and waterflow rate. Past Bureau work shows 
that a throat differential above 10 in H2O gauge would 
keep discharge concentrations below 2 mg/m in this 
installation (5). 

Owing to its simplicity and low cost, venturi-type 
scrubbers can be readily recommended for coal prep- 
aration plants. They are commercially available in a wide 



range of configiu-ations, including vertical (fig. 7). As a 
gmdeline, minimiun throat differential pressures should 
be 6, 10, and 15 in HjO gauge for respective inlet con- 
centrations of 10, 20, and 30 mg/m^ respirable dust. A 
venturi should not be operated at less than 4 in HjO gauge 
pressiu-e drop across the throat. 

ELECTROSTATIC PRECIPITATORS 

Electrostatic precipitators (5) pass airborne dust 
between electrically charged plates (fig. 8). This causes 
the dust to migrate toward and adhere to the plates; this 
dust is then automatically shaken or washed from the 
plates. Precipitators offer several advantages, including 
very-high dust collection efficiency, and very-low air 
pressure drop (1/2-in HjO gauge across the precipitator). 
This results in low fan power requirements. Initial, $1 to 
$1.50 per cubic foot per minute, and operating costs are 
lower than scrubbers or baghouses. 



Dusty 
air in 




-Drain 
Figure 7.-Venturl-type scrubber — vertical arrangement 



Disadvantages include fairly large in size, and a 
requirement for relatively experienced m2dnten2mce 
personnel to work with the high voltages, and potential 
sparking problems associated with high voltages. 
Efficiency is reduced if subjected to vibration, if inlet air 
velocity is not uniform, and if shutdown time is required 
for cleaning. Detergent water is frequently required for 
cleaning. 

Results of efficiency tests of electrostatic precipitators 
showed a wide efficiency difference between various plants. 
One 5,000 cfm installation was only 30 pet efficient on 
respirable coed dust. Minerals containing sodiimi can also 
present problems when high dust collection efficiency is 
required. For best results, electrostatic efficiency data on 
the specific dust and installation should be obtained before 
purchasing a precipitator. 



Gas 
passage 
between 
plates 




Dischorge 
electrode 



Hopper- 
Figure 8.-Electrostatic precipitator. 



CONTROL AT SOURCE 



For maximum cost effectiveness, dust should be 
controlled at its source prior to dilution and dispersal. 
Usually, dust sources are determined by visual observation, 
especially diuing startup or on-off procedures. Precise 
determination of the appropriate control technique de- 
pends on many factors, some beyond the scope of this 
report. However, these general guidelines should be 
followed: 

* A thorough dust survey should be conducted when 
it appeeu-s that control costs will be significant. 

* The survey should at least determine the dust type 
(if unknown), its sources, extent, and concentration. 

* The control technique should be primarily based on 
survey data. For example, simple ventilation may be 
suitable for a dust concentration of 4 mg/m^, but not 
for 10 mg/m^. 

Although not pointed out in most literature, high dust 
concentrations or low threshold limit value (TLV) usually 
require more severe and costly controls, such as baghouses 
or scrubbers. Frequently, two control techniques such as 
water sprays and ventilation can work together for maxi- 
mum cost effectiveness. 

WATER SPRAYS 

Where the product aillows, water sprays can be very cost 
effective for dust source control (6-7). Their primary 
effectiveness is in keeping the dust from becoming 
airborne, accordingly the effectiveness is primarily de- 
pendent on flow rate and coverage, not water pressure. 
Once airborne, respirable dust is very difficult to knock 
down. This knockdown effectiveness is dependent on flow 
rate emd pressure, and the best spray systems seldom 
exceed 60 pet efficiency on respirable dust. Although not 



generally recommended due to cost, water additives 
(surfactants) (8), foams (9), and steam (10) can occasion- 
cdly provide additional source control. 

GOOD HOUSEKEEPING 

Effective housekeeping is essential for maintaining 
acceptable dust levels. Product material allowed to 
buildup on the walls, beams, equipment, floors, and 
walkways can be readily dispersed. Dust buildup on grated 
floors and walkways is a significant dust contributor, 
especially with the higher floor levels; a worker can create 
a significant amount of dust just walking across this 
grating, causing dust to drop three or four floors. An 
effective housekeeping practice is to clean the mill or plant 
at least once a day. Many operations will wash down all 
floors and equipment to achieve this, although in some 
plants that do not allow for the use of water, vacuum 
cleaning systems are used. Sweeping or blowing clean air 
with compressed air should not be used. Figure 9 shows 
the increase in the respirable dust exposure to a worker 
when a co-worker in a mill was sweeping the floor one 
level below with a push broom. The operator's dust 
exposure increased to nearly six times the previous 
concentration because of this dust. 

Plants or mills with low dust concentrations generally 
had good housekeeping practices. Most of these opera- 
tions would wash down or vacuum the plant or mill each 
shift. This is not to suggest that effective housekeeping 
was the sole reason for low dust concentrations, but to 
emphasize that those operations who were trying to 
maintain low dust levels realized the importance of this 
practice. Various types of commercial vacuum cleaning 
devices are available for this purpose. They range from 
high capacity remote vacuum collectors (fig. 10) to easily 
portable tank-type units. 



to 

E 

E 



o 
o 

I- 




-'^t . 



TIME, min 

Figure 9.-lncrease in worlter's dust exposure from broom 
sweeping. 




To dust 
collector 



■^:>^mH^..:::: 



Figure lO.-IHigh capacity remote vacuum system. 



PERSONAL PROTECTION DEVICES 



Occasionally, dust control with baghouses or scrubbers 
may not be practical, as from various sources spread over 
large areas. In these cases, control at the dust sources is 
suggested. Where this is prohibitive due to costs, space 
limitations, or other reasons, use of the dust helmet or 
other personal protection devices such as respirators is 
strongly recommended. 

DUST HELMETS 

Dust helmets have been designed to provide dust 
protection for the wearer without some of the talking, 
spitting, and fit problems associated with typical respi- 
rators. They use a small fan in the hebnet to provide 
filtered air to the breathing zone of the wearer (fig. 11). 
The fan is powered by a rechargeable belt-mounted battery 
suitable for 8-h continuous operation. A high efficiency 
filter is fitted to the fan discharge, a coarse filter at its 
inlet; both are throw-away types. The helmet is used with 
a full-face lucite lens. Tests of the helmet's efficiency on 
various dusts and conditions generally show excellent 
results and reasonably good acceptance by personnel, 
except when they must work in tight places. Various types 
of side shields are available to increase the helmet's dust 
protection efficiency as ambient ak velocity rises. Bureau 
tests of the helmet with two types of side shields show dust 
reductions greater than 90 pet between ambient and inside 
helmet respirable concentrations (11). Typical in-plant 
tests would show 92 pet without a full-face side shield, and 
98 pet with. 

The hebnet can be very useful in extreme dust 
conditions, such as in coal preparation plcmts with high 
respirable concentrations or in mineral processing plzmts 



Main filter 



Safety helmet 
shell 



Motor and 
fan assembly 




Battery 
pack 



Hinged 
clear 
visor 



Figure 1 1 .-Dust helmet 




Figure 1 2.-Disposable-type respirator. 



with high siHca dust levels. Primary disadvantages are 
glare from the lens, especially for those who must wear 
glasses, and the excessive bulk for tight work spaces. 

RESPIRATORS 

Many types of dust respirators are also commercially 
available for this use. Dust protection efficiency is roughly 
dependent on cost, type, face fit, and ranges from 
approximately 60 pet for simple throw-away or negative- 
pressure types (fig. 12), to the upper 90 pet for powered 
positive-pressure types (fig. 13). When respirable silica 
dust concentrations exceed 5 mg/m^, or long-term ex- 
posure is anticipated, the latter types should be used. 
However, dust helmets and respirators only help protect 
the worker from usual dust and mist, they are not to be 
used for harmful chemical dusts or gasses. 




Figure 13.-Posltive pressure-type respirator. 



SPECIAL PROBLEMS 



PIPE AND DUCT CLOGGING 

Serious duct clogging problems occur in some of the 
duct collector systems in coal preparation plants; dust 
peirticles graduidly accumulate in the horizontal duct rims 
between the pickup points and fan, and zdmost completely 
block the airflow. This not only cripples the performance 
of the duct collector system, but also presents safety 
problems. The added weight of the dust can occasionedly 
break the duct hangers, allowing sections to drop. 
Clogging could occur as often as every 6 months. In these 
cases, the prepjiration plant operators would occasionally 
drop these duct sections and flush with high-pressure 
water. The common remedy for this problem is to 



maintain an adequate duct ak velocity as indicated in the 
American Conference of Government-Industrial Hygienist 
(ACGIH) Handbook, however, the duct air velocity in 
some plants is aheady above 4,000 ft/min. Conversations 
with material handling experts show that these problems 
are not rare, even in hi^-velocity systems. No simple 
solution has been foimd to the duct clogging problem. 
One possibility has been to paint the duct interior, first 
with an epoxy nonlift primer followed by an epoxy topcoat, 
both with suitable catalyst. Extra water sprays in the duct 
sections that clog have also been successful. In some 
cases, a dropout box or inertial separator can help. Use 
of inertial separators can also help prevent scrubber 
clogging. Inertifd separators separate large dust particulate 



10 



from the airstream xising a centrifugal, gravitational, or 
inertial force. The separate dust faiUs into a hopper, where 
it is temporarily stored. The three primary types are 
settling chambers, baiffle chambers, and centrifugal 
collectors. Neither settling chambers nor baffle chambers 
are commonly used in the minerals processing industry, 
mostly due to space requirements, and difficulties with 
cleaning and disposal of the collected dust. 

Settling chambers (fig. 14) consist of a large box 
installed in the ductwork. The sudden enlargement at the 
chamber reduces the speed of the dust-filled airstream, 
and heavier particles settle out. They are simple in design 
and can be readily manufactured. 

Baffle chambers (fig. 15) use a fixed baffle plate that 
causes the airstreeun to make a sudden change of direction. 
Large-diameter particles do not follow the gas stream, but 
continue into a dead air space and settle. 

Centrifugal collectors (fig. 16) use cyclone-type action 
to separate dust particles from the air. In a typical 
cyclone, the high velocity dusty air enters at a tangent. 
The centrifugal force created by the circular flow forces 



Dusty 
air inlet 




Outlet 



Dust collection 
hoppers 

SETTLING CHAMBER 
Figure 14.-Settling type drop-out chamber. 




the dust particles toward the wall of the cyclone, where 
they fall into a hopper. These collectors are more 
expensive and more efficient in removing particulate than 
settling and baffle chambers. 

CONTROL ROOM DUST 

Control room dust is generally not a serious problem, 
except when the dust contains silica. Sihca is not a 
problem in coal preparation plant control rooms, but it can 
be a serious health problem in many mineral processing 
plant rooms, especially because of the long-term, 8-h 
occupjmcy. Dust can Jilso be detrimental to electrical 
equipment. 

Wall- or window-mounted air conditioners admitting 
filtered outside air are adequate for control in most coal 
preparation plants. For mineral processing plants, high 
efficiency filters are strongly recommended. Minimum 
makeup airflows should be above several hundred cubic 
feet per minute even for small rooms (say 1,000 ft^); this 
air should not be recirculated, but allowed to leak from the 
room. This will help insure a positive pressure within the 
room, and reduce dust infiltration. Conventional through- 
the-wall or window-type ziir conditioners should not be 
used for room pressurization; their makeup (fresh) airflow 
is usually too low for mineral processing plants. 



Inner vortex 



To hopper 
BAFFLE CHAMBER 




Outer vortex 



hopper 
CENTRIFUGAL TYPE 



Figure 1 5.-Baffle type drop-out chamber. 



Figure 1 6.-Centrifugal type drop-out chamber. 



11 



CONCLUSIONS 



Currently, about one-third of the coal preparation in both cases are essentially indentical and depend on dust 



plants in the U.S. have excessive respirable dust concen- 
trations in specific areas. Results from extensive work at 
mineral processing plants with dust problems indicate that 
dust concentrations are lower, but frequently compoimded 
by sihca and full-shift exposure. The primary solutions 



soiu"ces, type, and concentration. In broad order of practi- 
cahty and cost-effectiveness, these solutions are ventilation, 
baghouses, scrubbers, good housekeeping, and personal 
protection devices. 



REFERENCES 



1. Williams, K. L., and R. J. Timko. Performance Evaluation of a 
Real-Time Aersol Monitor. BuMines IC 8968, 1984, 20 pp. 

2. Grigal, D., G. Ufken, J. Sandstedt, M. Blom, and D. Johnson. 
Development of Improved Scrubbers for Coal Mine Applications 
(contract H0199055). BuMines OFR 91-83, July 1982, 124 pp.; NTIS PB 
83-205385. 

3. Divers, E. F., and J. T. Janosik. Scrubbers for Dust Control: A 
Comparison of Six Medium-Energy Use Types. BuMines RI 8449, 1980, 
29 pp. 

4. . Comparison of Five Types of Low-Energy Scrubbers for 

Dust Control. BuMines RI 8289, 1978, 38 pp. 

5. McDonald, J. J., and A. H. Dean. Electrostatic Precipitator 
Manual, Noyes Data Corp., 1982, 480 pp. 

6. Mody, v., and R. Jakhete. Conveyor Belt Dust Control (contract 
H0113007, Martin-Marietta Lab.). BuMines OFR 31-86, Feb. 1984, 
410 pp. 



7. Mody, v., and R. Jakhete. Dust Control Handbook for Minerals 
Processing (contract J0235005). BuMines OFR 2-88, Feb. 1987, 220 pp.; 
NTIS PB-88-159108. 

8. Volkwein, J. C, A. B. Cecala, and E. D. Thimons. Moisture 
Application for Dust Control. Appl. Ind. Hyg., v. 4, No. 8, Aug. 1989, 
pp. 198-200. 

9. . Use of Foam for Dust Control in Minerals Processing. 

BuMines RI 8808, 1983, 11 pp. 

10. Cecala, A. B., J. C. Volkwein, and E. D. Thimons. Adding Steam 
To Control Dust in Mineral Processing. BuMines RI 8935, 1985, 9 pp. 

11. Cecala, A. B., J. C. Volkwein, E. D. Thimons, and C. W. Urban. 
Protection Factors of the Airstream Helmet. BuMines RI 8591, 1981, 
17 pp. 



INT.BU.OF MINES,PGH.,PA 29136 



■0 

m 




Si o 


5 W 


z 




"> ZI 


m ■ 


> 

5 


o 

■n 
■n 


3" 


C <B 
©■p. 


■71 

o 


O 

> 


treet 
ton, 




"V 


r- 


0- 


5 «> 


a 

1 


00 

c 

V) 







m 

c 

V) 


z 
m 


S2 


3" 

(D 


V) 




3 






(O 


n 


i 




00 


^ 


® 







5' 









^ 



> 
z 
m 
D 
c 
> 

o 

"D 
TJ 
O 
3) 



m 



O 

-< 
m 
3) 



413-90 

















'^o^ 
























< * ^.^ * • • - -a 





^ "■• ^V^ '^ '•-» 



* A <^ ♦'TV.- 4 



^•\ «c°^;^^% /^-^'X .^<*;^'>- X'-^i'X c^ *:^^'> .^^ 








S°^ 



...i^m^. ^^^^^^^^ y^^^ t^^^^<i> ,»^^\ %^^^^' 








4ECKMAN 

"NDERY INC. 



• ■ o ' 



"^ '•-^K*' ^^^^'\ °''^^^*' •j.'^^^ ■"• 



'%.n<i^ *^^^ ^r.'^ 

<?5 °^ 



0^ ^^/^TT,.* ^^^'^ -o^' 



^^ N. MANCHESTER, -* ♦><^* tV K'i 'f^V A. -A '^^S^K'^ .'V ^-. 'X^T* A* ^^ "^'^i^J^* 







